1. Field of the Invention
The present invention relates to a gas detection apparatus which detects the concentration of a gas present in an object atmosphere.
2. Description of the Related Art
In recent years, in order to satisfy social needs such as protection of the environment and nature, research has actively been conducted on fuel cells which are efficient and clean energy sources. Among them, polymer electrolyte fuel cells (PEFC) and hydrogen internal combustion engines are expected as energy sources for homes, vehicles, etc., because they can operate at low temperature and have a high output density.
In these systems, detection of gas leakage is important because these systems use hydrogen, which is a combustible gas, as fuel.
A combustible gas detection apparatus has been known which detects the concentration of such a combustible gas present in an object atmosphere. The known combustible gas detection apparatus includes a gas detection element which is disposed in an object atmosphere and which includes a heat generation resistor and a temperature measurement resistor provided thereon. The resistance of the heat generation resistor changes due to a change in the temperature of the heat generation resistor itself (due to heat generation of the heat generation resistor). The resistance of the temperature measurement resistor changes due to a change in environmental temperature.
Specifically, in this combustible gas detection apparatus, the resistance of the heat generation resistor in the gas detection element is controlled using a bridge circuit such that the resistance alternately becomes equal to resistance values corresponding to two set temperatures (first and second set temperatures). Further, the concentration of combustible gas is calculated from control voltages (voltages across the heat generation resistor) at that time and a voltage difference (temperature voltage) produced as a result of a change in the resistance of the temperature measurement resistor.
The switching between the two set temperatures (first and second set temperatures) is performed, for example, by alternately selecting fixed resistors which are provided in the bridge circuit and have different resistance values every time a predetermined period of time has elapsed (see, for example, Patent Documents 1 and 2). As a result, by using one bridge circuit and a heat generation resistor, the voltages across the heat generation resistor at the first and second set temperatures can be detected, whereby the size of the gas detection element can be decreased, and power consumption can be suppressed.
Also, according to Patent Documents 1 and 2, considering that the concentration of combustible gas contained in an object atmosphere changes with the humidity of the object atmosphere, not only the above-described voltage difference, but also the ratio (voltage ratio) of the voltage generated across the heat generation resistor at the first set temperature to the voltage generated across the heat generation resistor at the second set temperature is computed. Further, the gas concentration is corrected by making use of the fact that this ratio is approximately proportional to the humidity.
Meanwhile, in the above-described combustible gas detection apparatus, the gas detection element is accommodated in a casing member having a gas inlet opening. Further, the object atmosphere is allowed to flow into the casing member and flow out of the casing member through the gas inlet opening, whereby detection by the gas detection element is enabled. However, if dirt, water, oil, or the like adheres to the gas inlet opening and the gas inlet opening is clogged (clogging), detection of the gas becomes inaccurate or impossible. Therefore, a determination as to whether or not the gas inlet opening is clogged must be made.
In view of the above, a technique has been developed for determining whether or not the gas inlet opening is clogged (see Patent Document 3). According to this technique, a monitor (air flow monitor) which measures the flow of air is disposed within the casing member, and a determination as to whether or not the gas inlet opening is clogged is made based on a change in the flow rate of air.
[Patent Document 1] Japanese Patent No. 4302611
[Patent Document 2] Japanese Patent Application Laid-Open (kokai) No. 2014-20859
[Patent Document 3] Japanese Patent Application Laid-Open (kokai) No. 2005-52833